In the 1950's it was discovered that enzootic abortion in ewes (EAE) was carried by infection of the animals with a virus strain of the Psittacosis-Lymphogranuloma group [Young et al, J.A.V.M.A., Vol. 133 (Oct. 1, 1958) p. 374] was classified as Chlamydia sp. [Becerra et al, Zbl. Bakt., I. Abt. Orig. Vol. 214, pp. 250-258 (1970)].
Earlier in the same decade, workers in the field found that the inoculation of ewes at or near breeding time with a vaccine prepared from (1) virus infected yolk sacs or (2) virus infected ovine foetal membranes was effective in reducing the incidence of EAE. The chlamydia elementary bodies were concentrated from infected chicken embroys or foetal membranes, inactivated and used to prepare the vaccine. McEwen et al, Vet. Rec., Vol. 63, p. 197 (3/17/51); McEwen et al. Vet. Rec., Vol. 66, p. 393 (7/10/54); McEwen et al. Vet. Rec., Vol. 67, p. 393 (5/21/55); McEwen et al, Vet. Rec., Vol. 68, p. 686 (10/6/56); McEwen et al, Vet. Rec. Vol. 68, p. 690 (10/6/56); Hulet et al, Am. J. Vet. Res. Vol. 26, p. 1464 (1965); Frank et al, Am. J. Vet. Res., Vol. 29, p. 1441 (1968); Meinershagen et al, Am. J. Vet. Res., Vol. 32, p. 51 (1971).
Serial passage of a virulent microorganism outside the natural host is an accepted method for selecting variants (attenuated strains) with decreased virulence. Wilson, G. S., and Miles, A. Topley and Wilson's Principles of Bacteriology, Virology, and Immunity, Williams & Wilkins, 6th edition 1975 Vol. 1, p. 412-416.
There have also been reports of successful immunizations of ewes against chlamydial abortion with chlamydial organisms attenuated by serial passage in chicken embroys. See Mitscherlich, E., The Control of Virus Abortion of Sheep, Berl-Munch. Tierarztl, Wschr. 78, Heft 5: 81-100 (1965); Nejvestic, A. and Forsek, Z., Active Immunization in the Prophylaxis of Enzootic Abortion in Ewes--I. Vet. Glasnik, 23, 6: 423-427 (1969); Schoop, G., Wachendorfer, G., Kruger-Hansen-Schoop, U., and Berger, J., Studies on a Live Vaccine for the Control of Miyagawanella Abortion in Sheep, Zbl. Vet. Med., Reihe B., 15, Heft 2: 209-223 (1968); Yilmaz, S., and Mitscherlich, E., Experiences in the Control of Ovine Enzootic Abortion with a Live Vaccine made from an Attenuated Strain of Chlamydia ovis, strain "P", Berl. Munch., Tierarztl., Wschr. 86, Heft 19: 361-366 (1973). Other reports, however, indicate that the virulence of chlamydial organisms was not diminished by serial passage in cell culture or in chicken embryos. See Becerra, V. M., Ata, F. A., and Storz, J., Studies on the Response of Ewes to Live Chlamydia Adapted to Chicken Embryos or Tissue Culture, Canad., J. Compar. Med. 40: 46-52 (1976); Becerra, V. M., and Storz, J., Tissue Culture Adaptation and Pathogenic Properties of an Ovine Chlamydial Abortion Strain, Zentbl, Vet. Med 21: 288-299 (1974); and McKercher, D. G., Robinson, E. A., Wade, E. M., Saito, J. K. and Franti, C. E., Vaccination of Cattle against Epizootic Bovine Abortion, Cornell Vet. 59: 211-226 (1969).
The cost of producing a vaccine utilizing infected yolk sac and/or ovine foetal membrane, however, is prohibitive. Moreover, the vaccine produced according to the published methods is difficult to standardize. The yolk sac and foetal membrane techniques are not only expensive but are time-consuming and inefficient. As a result, there is no commercially attractive EAE vaccine available in the United States at the present time. It is believed that immunizations against EAE are presently carried out only in Idaho utilizing locally batch-produced yolk sac or foetal membrane based vaccines. The difficulty of standardization of these vaccines, however, has led to mixed and unpredictable results.
Inasmuch as enzootic abortion in ewes (EAE) is a serious problem in the sheep-breeding industry and responsible for abortion losses in susceptible flocks as high as 30%, there is an urgent need for an economically viable vaccine and method for the active immunization of ewes against EAE.
It is an object of the present invention to provide a vaccine for the active immunization of ewes against enzootic abortion.
It is a further object of the present invention to provide a method for immunizing ewes against EAE.
It is a further object of the present invention to provide a relatively simple, efficient and economically attractive method for the preparation of a vaccine against EAE.
It is a still further object of the present invention to provide an easily standardized intermediate composition especially adapted for preparing a vaccine against EAE.